Recently, liquid crystal display devices are being more widely used in thin, high resolution display devices. A typical schematic view of such liquid crystal device is shown in FIG. 1. The liquid crystal display device comprises a backlight 1, a liquid crystal cell 400, and polarizing plates 402a and 402b. The liquid crystal cell is disposed on the light emitting surface of the backlight. Light emitted from the backlight 1 toward the liquid crystal cell 400 (arrows) is transmitted through a polarizing plate 402a, and a glass substrate 403 on which a TFT 405 and a wiring layer 404 are formed, and enters into a liquid crystal layer 410 in which liquid crystal 409 is enclosed. The orientation of the liquid crystal layer is controlled by the TFT 405 and the wiring layer 404 both formed on the glass substrate 403. That is, the TFT 405 and the wiring layer 404 are connected to a driver (not shown), which supplies a predetermined voltage to each pixel through the wiring layer 404 and the TFT 405. Depending on this applied voltage, the liquid crystal molecule is oriented to a predetermined extent. The light transmitted through each pixel is modulated by this orientation. The modulated light is transmitted through an upper glass substrate 406, and is shielded or transmitted by the polarizing plate 402b. This determines transmissivity for each pixel, so that graphics and/or characters can be displayed in a meaningful pattern. The upper glass substrate 406 is formed with a black matrix layer and a color filter layer (not shown).
Such a liquid crystal display device has problems with its angle of view, its response, and the magnitude of its driving voltage. That is, one of problems of the liquid crystal display device is that variation or reversal of the transmissivity occurs with changes in the angle of view. (The angle of line of sight from the normal of screen.) If variation or reversal of the transmissivity occurs depending on the angle of view, an area which should appear blue can appear as if it is red, so that an original image is not reproduced accurately. The angle of view for a conventional twisted nematic (TN) type liquid crystal display is as narrow as about .+-.40.degree. in the horizontal direction and about .+-.15-30.degree. in the vertical direction.
In addition, when a driver of the liquid crystal display device drives the TFT, if the driving voltage is too high, it imposes a load on the driver and requires the use of an expensive driver. This significantly affects the life and cost of a liquid crystal display device. A conventional liquid crystal device typically uses a driving voltage of 5 V or less.
An approach for improving the angle of view in a TN type liquid crystal is an arrangement in which, as shown in FIG. 2, an optical phase difference compensating film 420 is placed between the glass substrates 403 and 406, and the polarizing plate 402a and 402b. For example, Japanese Published Unexamined Patent Application (PUPA) No. 2-91612 discloses a TN type liquid crystal display device in which compensating means is placed between a polarizing means and a glass substrate. Similarly, PUPA 6-75116 discloses a similar arrangement in which the optical axis with the minimum refractive index of optical phase difference compensating means is along a direction parallel to the plane, and the direction of one of other optical axes is inclined with respect to the normal of the plane. PUPA No. 5-313159 discloses a similar arrangement in which only one of refractive indices on the x, y and z axes of optical phase difference compensating means is high, while the other two are identical. All of these inventions relate to a TN type liquid crystal display device which differs from the display device of the present invention.
Using a compensating film cannot completely compensate the modulation occurring in the TN type liquid crystal layer. This is because the liquid crystal molecules themselves are twisted in the TN type liquid crystal layer, and the compensating film cannot compensate the optical effect on the twisting in view of its structure. In addition, a significant problem is imposed by reversal of gradation in the TN type liquid crystal layer.
An approach to improve the angle of view is to use a liquid crystal cell with bend orientation. The using of the liquid crystal cell with bend orientation is called OCB (Optically Compensated Birefringence). The bend orientation (also called a .pi. cell) has an orientation which can be optically self-compensated, and is defined, in various publications, it has an orientation plane symmetrical with respect to a center plane 83 between substrates 81 and 82. As shown in FIG. 3, that twist does not substantially exist in a plane parallel to the glass substrates 81 and 82. It has arc continuity in which the angle .theta. between the major axis of the liquid crystal and the glass substrate is substantially 0.degree. (or, 90.degree.) near the lower glass substrate, gradually increasing (decreasing) toward the center line, to about 90.degree. (0.degree.), then gradually decreasing (increasing) toward the upper glass substrate, and there is substantially 0.degree. (90.degree.). The bend orientation has advantages that it has good response, and can be easily compensated for optical phase difference, so that the angle of view can be increased.
Conventionally, a negative birefringence film as in PUPA 6-294962 (U.S. Pat. No. 5,410,422), or an optically biaxial film described in "Improvement of Gray-Scale Performance of Optically Compensated Birefringence (OCB) Display Mode for AMLCDs," C. L. Kuo, et. al., Tohoku University, DIS '94 Digest have been used for the optical phase difference compensation for a bend oriented liquid crystal layer. Both of them can provide perfect optical phase difference compensation in the horizontal angle of view, but cannot provide a sufficient angle of view in the vertical or oblique direction because it cannot compensate the optical activity of the liquid crystal layer.
As described in "Optical Design of an OCB Display Device," Hajime Nakamura, Japanese Telecommunication Society Technical Bulletin, EID-95-115 (2/20/96), or "An Optimization of OCB Optics," H. Nakamura, IBM Japan Ltd., SID '96 Digest, there is an approach to significantly improve the vertical angle of view characteristics. This involved reducing .DELTA.nd (retardation), which is a product of index anisotropy (.DELTA.n) of the liquid crystal layer and its thickness (d), to decrease the optical activity of the liquid crystal layer. However, since .DELTA.nd of the liquid crystal layer is small. This approach is not always suitable for a portable notebook personal computer which requires a high transmissivity.
In addition, since the conventionally contemplated OCB type liquid crystal display device uses a high driving voltage as 6-10 V, design of a driving system is difficult while the driving voltage of normal twisted nematic liquid crystal is 5 V or less.